
Calamansi purée is a citrus product of pulped calamansi, a sour mandarin-kumquat hybrid, for souring and flavor.
Sour, with an orange-peel aroma over the top, and that aroma is the reason to use it. It comes frozen or bottled far more often than fresh, so check whether yours is sweetened before you balance the drink. Rum, gin and honey take it well.
Calamansi juice
Swaps straight in. Purées use the whole fruit, so strained juice without the peel oil will seem less aromatic.
Lemon mix
Blend 3 parts lemon juice with 1 part mandarin. This mix acts as a one-for-one swap, with about the same acid.
Hits the right acid level but lacks the mandarin note. Swap it for most of the measure, and make up the rest with mandarin or orange juice.
Fresh keeps a couple of days refrigerated and freezes cleanly, which is how most bars hold it. Frozen in an ice tray it goes straight into a shaker by the cube.